1. Technical Field
The present invention relates to radio frequency (RF) transport systems and, more specifically, to a RF transport system capable of efficiently transporting RF signals from one or more RF transceivers which employ frequency hopping.
2. Related Prior Art
Wireless communications services using RF signals became readily accessible to the general public with the advent of cellular radio systems. In a typical cellular radio system a geographical area (e.g., a metropolitan area) is divided into several smaller, contiguous radio coverage areas, called "cells", which are served by a corresponding group of fixed radio stations, called "base stations", each of which includes a plurality of radio frequency (RF) channel units (transceivers) that operate on a subset of the RF channels assigned to the system, as well known in the art. The RF channels allocated to any given cell may be reallocated to a distant cell in accordance with a frequency reuse plan as is also well known in the art. In each cell, at least one RF channel, called the "control" or "paging/access" channel, is used to carry control or supervisory messages. The other RF channels are used to carry voice conversations and thus are called the "voice" or "speech" channels.
The cellular telephone users (mobile subscribers) in the aforementioned system are usually provided with portable (hand-held), transportable (hand-carried) or mobile (car-mounted) telephone units, collectively referred to as "mobile stations", each of which communicates with a nearby base station. Each of the mobile stations includes a microphone, a loudspeaker, a controller (microprocessor) and a transceiver, as well known in the art. The transceiver in each mobile station may tune to any of the RF channels specified in the system (whereas each of the transceivers in the base stations usually operates on only one of the different RF channels used in the corresponding cell).
The base stations in the aforementioned system are connected to and controlled by a mobile telephone switching office (MTSO) which, in turn, is connected to a local central office in the landline (wireline) public switched telephone network (PSTN), or to a similar facility such as an integrated services digital network (ISDN). The MTSO switches calls between wireline and mobile subscribers, controls signalling and assignment of voice channels to the mobile stations, performs "handoffs" of calls from one base station to another, compiles billing statistics, stores subscriber service profiles, and provides for the operation, maintenance and testing of the system.
The original cellular radio system, as described generally above, used analog transmission methods, specifically frequency modulation (FM), and duplex (two-way) RF channels in accordance with the Advanced Mobile Phone Service (AMPS) standard. In the United States, this original AMPS (analog) architecture formed the basis for an industry standard sponsored by the Electronics Industries Association (EIA) and the Telecommunications Industry Association (TIA), and known as EIA/TIA-553. In the middle to late 1980s, however, the cellular industry both in the United States and in other parts of the world began migrating from analog to digital technology, motivated in large part by the need to address the steady growth in the subscriber population and the increasing demand on system capacity. The industry thus developed a number of air interface standards which use digital voice encoding (analog-to-digital conversion and voice compression) and advanced digital radio techniques, such as time division multiple access (TDMA) or code division multiple access (CDMA), to multiply the number of voice circuits (conversations) per RF channel (i.e., to increase capacity).
In Europe and Japan, the GSM and PDC standards, respectively, both of which use TDMA, have been widely implemented. In the United States, the EIA/TIA has developed a number of digital standards, including IS-54 (TDMA) and IS-95 (CDMA), both of which are "dual mode" standards in that they support the use of the original AMPS analog voice channels (AVCHs) and analog control channel (ACCH), in addition to newer digital traffic channels (DTCHs) defined within the existing AMPS framework, so as to ease the transition from analog to digital and to allow the continued use of existing analog mobile stations. The dual-mode IS-54 standard, in particular, has become known as the digital AMPS (D-AMPS) standard. More recently, the EIA/TIA has developed a new specification for D-AMPS, which includes a digital control channel (DCCH) suitable for supporting various data services, sometimes referred to as "personal communications services" (PCS), and extended mobile station battery life. This new specification, which builds on the IS-54B standard (the current revision of IS-54), is known as IS-136.
Along with the emergence of digital cellular and PCS, there has been a trend towards the integration of telephone and data services with television (TV), computer and/or multimedia networks. FIG. 1 shows a typical RF transport system (inside dashed box) which interconnects a cellular or PCS radio base station (RBS) 10 with a mobile station (MS) 20. The RF transport system comprises a central transport unit 12, a RF transport network 14 and a remote transport unit 16. The central transport unit 12 receives a RF signal on a first frequency f.sub.x from the RBS 10 a nd converts that signal into a signal at a second frequency f.sub.y suitable for transmission over the RF transport network 14. Depending o n the application, the RF transport network 14 may comprise, for example, a local area network (LAN), a wide are a network (WAN), the global communications network known as the Internet, a wired or "wireless" cable TV network, a video network, a fiberoptic network or a point-to-point microwave network. The signal that is carried through the RF transport network 14 at frequency f.sub.y is finally provided to th e remote transport unit 16 which converts this signal into a signal at a third frequency f.sub.z for transmission through an antenna 18 to the MS 20.
The use of RF transport systems as generally depicted in FIG. 1 is complicated in practice by the use of frequency "hopping" at the base station. Some cellular or PCS systems, such as those which implement the GSM standard, vary (hop) the frequency of the signal transmitted from the base station to the mobile station over time in order to reduce the deteriorative effects of Rayleigh fading (the phenomena wherein the received signal strength will vary due to multipath propagation of the transmitted signal). By rapidly changing the frequency of the transmitted signal from the base station, the fading locations will vary over the course of a call, thus decreasing the average depth and duration of fading dips at the mobile station. Of course, the receiver in the mobile station must hop along with the transmitter in order to correctly receive the signal. For this purpose, synchronization information regarding the relevant hopping sequence is usually transmitted from the base station to the mobile station over a broadcast or dedicated control channel.
FIG. 2 illustrates the use of frequency hopping at the RBS 10 shown in FIG. 1. The RBS 10 includes a plurality of transceivers 11 such as transceivers 1 . . . 5. One of the transceivers (e.g., transceiver 1) in the RBS 10 is used for control channel signalling and is assigned a fixed frequency f.sub.1. Each of the other transceivers (e.g., transceivers 2 . . . 5) in the RBS 10, on the other hand, hops within a predefined set of frequencies such as f.sub.2 -f.sub.5 using a unique hopping sequence that defines the order of the frequencies at its output over time. For example, in a TDMA system wherein the hopping sequence repeats every four bursts, the outputs of the frequency hopping transceivers (transceivers 2 . . . 5) may be as shown in FIG. 3.
When transmitting the various outputs of the frequency hopping transceivers (transceivers 2 . . . 5) through the RF transport network 14, it is desirable that the various output frequency signals be "packed" together so as to make efficient use of the available bandwidth in the network 14, and that these packed signals be translated into signals in some predefined area of the spectrum such that they can coexist with other RF signals (e.g., cable TV or satellite signals) that are being simultaneously transmitted over the network 14. Upon exiting the RF transport system, these packed and translated signals may be "unpacked" and translated back to their original frequencies for transmission through the antenna 18 to the MS 20. Current implementations of RF transport systems, however, do not allow for such desired packing of the hopping frequencies. Rather, these systems use so-called "block conversion" in which a block of hopping frequencies from a transceiver is converted into an equal block of frequencies in a different part of the spectrum that is suitable for transmission over the network 14, without any packing of frequencies. This approach clearly wastes valuable bandwidth in the transport network.